.circle{
    width: 50px;
    height: 50px;
    border-radius: 50%;
    display: inline-block;
    text-align: center;
    line-height: 50px;
    color: white;
    border: 2px solid #808080;
}
.circle:nth-child(2n){
    background-color: red;
}
.circle:nth-child(2n+1){
    background-color: blue;
}